Transducers generally convert electrical signals to mechanical signals or vibrations, and/or mechanical signals or vibrations to electrical signals. Acoustic transducers in particular convert electrical signals to acoustic signals (sound waves) in a transmit mode (e.g., a speaker application), and/or convert received acoustic waves to electrical signals in a receive mode (e.g., a microphone application). The functional relationship between the electrical and acoustic signals of an acoustic transducer depends, in part, on the transducer's operating parameters, such as natural or resonant frequency, acoustic receive sensitivity, acoustic transmit output power and the like.
Transducers, such as ultrasonic transducers, are provided in a wide variety of electronic applications, including filters. As the need to reduce the size of many components continues, the demand for reduced-size transducers continues to increase, as well. This has lead to comparatively small transducers, which may be micromachined according to various technologies, such as micro-electromechanical systems (MEMS) technology.
Various types of MEMS transducers, such as piezoelectric micro-electromechanical ultrasonic transducers (PMUTs), include a resonator stack, having a layer of piezoelectric material between two conductive plates (electrodes), formed on a thin membrane. To provide stable and predictable operation, the membrane is typically designed to have a net tensile stress. The operating characteristics of the resonator stack such as the operating frequency and amplitude are dependent on the tensile stress and in-plane stress.
MEMS transducers typically comprise the membrane provided over a die substrate. The die substrate is then provided on a substrate package using an adhesive material such as known epoxy or similar resin. The die substrate is thereby bonded to the package substrate, which may be provided in further packaging. When the epoxy cures shrinkage can occur. This shrinkage can impart additional stress on the die substrate and ultimately to the membrane of the MEMS transducer. Because the operating characteristics of the MEMS transducer are impacted by the stress on the membrane, the operating characteristics of the MEMS transducer can change, and in an unpredictable manner.
Additionally, during operation, the transducer package is subject to changes in temperature from both the environment and from the operation of the MEMS transducer itself. These changes in temperature can cause expansion and/or contraction of both the die substrate and the package substrate. Because of differences in the thermal expansion coefficients of the materials used for the die substrate and the package substrate, the die substrate and package substrate contract and/or expand unequally and, possibly unevenly. As such, additional stress can be imparted on the die substrate and ultimately on the membrane of the MEMS transducer due to temperature variations. Again, because the operating characteristics of the MEMS transducer are impacted by the stress on the membrane, the operating characteristics of the MEMS transducer can change, and in an unpredictable manner.